The present application relates to an optical element with an antireflection function and a method for manufacturing the optical element. More specifically, it relates to an optical element including structures formed from convex portions or concave portions and arranged in large numbers on the surface with a minute pitch smaller than or equal to the wavelength of the visible light, and a method for manufacturing the optical element.
Hitherto, regarding some optical elements by using light-transmitting substrates, e.g., glass and plastic, a surface treatment is performed to suppress surface reflection of light. As for this type of surface treatment, there is a treatment, in which fine and dense unevenness (moth-eye) is formed on an optical element surface (refer to “OPTICAL AND ELECTRO-OPTICAL ENGINEERING CONTACT”, Vol. 43, No. 11 (2005), 630-637, for example).
In general, in the case where a periodic uneven shape is disposed on an optical element surface, diffraction occurs when light passes through there, and a straight-ahead component of the transmitted light is reduced significantly. However, in the case where the pitch of the uneven shape is smaller than the wavelength of the transmitting light, diffraction does not occur. For example, when the uneven shape is rectangular, as described later, an effective antireflection effect can be obtained with respect to a single-wavelength light corresponding to the pitch, the depth, and the like.
As for a moth-eye structure produced by using electron beam exposure, a moth-eye structure in the shape of fine tents (pitch of about 300 nm, depth of about 400 nm) has been disclosed (refer to NTT Advanced Technology Corporation, “Master mold for anti-reflective structure (moth-eye) independent from wavelength”, [online], [searched on Feb. 27, 2008], internet <http://keytech.ntt-at.co.jp/nano/prd_0033.html>, for example). Regarding this moth-eye structure, a high-performance antireflection characteristic exhibiting a reflectance of 1% or less can be obtained.
Furthermore, as a moth-eye structure produced by using a method based on combination of an optical disk stamper producing process and an etching process, a moth-eye structure in the shape of a temple bell or the shape of an elliptical truncated cone have been disclosed (refer to International Patent Publication WO 08/023816, for example). Regarding this structure, an antireflection characteristic close to that based on the electron beam exposure is obtained.